What are referred to as shunt resistors are known for current measurement, said shunt resistors having narrow tolerated and temperature stable resistance values. The measurement current flows through these shunt resistors so that a voltage, which is proportional to the current, can be measured at the resistance. This voltage can be fed to the evaluation unit, e.g., a microcontroller, through an amplifier of the evaluation unit. In order to keep the ohmic losses in the shunt resistor low, the resistance value of shunt resistors is kept very low, in particular below 1 ohm. As a result, the voltage at the shunt resistor is also low, e.g., some mV. This low measurement voltage becomes a problem if the current measurement occurs at a potential that differs significantly e.g., by more than 50 V, from that of the evaluation unit. In this case, simple differential amplifiers have a common mode fault that would strongly falsify the measurement voltage. As a result, expensive special component parts, e.g., what are referred to as instrumentation amplifiers, must be utilized.
Instrumentation amplifiers may be obviated if the measurement current does not flow directly through the shunt resistor but instead through a current transformer so that the shunt can be set to the potential of the evaluation unit. This solution however is only possible for pure alternating currents.
If direct currents are also to be measured, current converters are often utilized, which evaluate the magnetic field generated by the measurement current, e.g., with a Hall sensor. Such type current converters are very expensive.
It is known to let a current, which is proportional to the measurement current, flow from the point of measurement to the point of evaluation, e.g., to the processor. Integrated circuits for measuring the current, which work according to this principle, are available. Generally, these circuits however can only measure one current direction so that two ICs are needed to measure currents the algebraic sign of which is unknown. This measurement electronics moreover needs a current supply at the potential of the point of measurement, which may however be considerably different from the potential at the point of evaluation. As a result, measurement is difficult.
On photovoltaic inverters in photovoltaic power systems with photovoltaic generators, an additional short-to-ground may occur in the event of a fault. In case of such a short-to-ground, a current flows through the grounding apparatus. This current must be detected if one wants to eliminate this short-to-ground condition.
A known method consists in measuring the current via a current converter. These converters are expensive though. Moreover, they comprise quite high an offset that leads to measurement problems.
On photovoltaic power systems, one also encounters the problem that the generator may be grounded e.g., at the positive pole whilst the processor or the signalling control system (SCS) of the inverter are connected to the negative pole of the generator. As a result, the potential of the point of evaluation may differ significantly, e.g., by some 100 volt, from the potential of the point at which the grounding current is measured.
Another difficulty is that there are different photovoltaic modules which require different grounding variants for best operation. On some commercially available modules, it is advisable to ground the generator at the positive pole whilst on others it is provided to ground the generator at the negative pole. A current to be measured can thus flow at two different points of a power system. This problem has been solved hitherto by the fact that a separate measurement channel, for example of a microprocessor, is provided for each point. As a result, it is necessary to constantly sample several channels if it is not known at which point a current flows.
It is also known to measure an insulation resistance Riso on ungrounded photovoltaic power systems. Such a method is known from DE 10 2006 022 686 A1 for example.
The document EP 1 857 825 A describes a measurement arrangement for a photovoltaic power system with an inverter. In order to determine an insulation resistance Riso, one uses two resistors Rs and two switches. If one of the switches is closed, the voltage is measured above the respective resistor Rs. A grounding current flows through the insulation resistance Riso. In order to measure insulation faults both between the positive pole and ground and between the negative pole and ground, which may occur at the same time, a resistor, i.e., two resistors in all, are utilized at different points.
The principle of a current mirror circuit is also known.
A current mirror circuit is known from FR 2 856 856 A. It serves for a measurement method for a battery charger.
The document GB 2 272 300 A teaches to utilize one single shunt with a current mirror circuit incorporating two transistors. This circuit serves to measure a small voltage drop in the event of a much higher supply voltage.
Another current mirror circuit is known from the document WO 03/052433 A, said circuit also being provided with one single shunt. The circuit is intended to comprise an offset setting means that is provided for imposing an offset of the output voltage.
Another mirror circuit is shown and described in U.S. Pat. No. 5,498,984 A. This circuit has an operational amplifier and a shunt. In this solution, a current may flow on either side when the voltage is supplied by a battery.
A current mirror circuit is also known from DE 198 44 465 A. Therein it is defined that a current mirror circuit consists of a reference branch with a semiconductor and of a mirror branch. The document JP 11160368 also discloses a mirror circuit with semiconductors.
A current mirror circuit for measuring a grounding fault on an amplifier is known from JP 56 006508 A. The circuit incorporates four transistors and two shunts at different points.